This invention relates to a method and means for drilling generally horizontal bores under roadbeds and the like.
More specifically, this invention relates to a pneumatic rotary-impact drill utilizing a drilling fluid and to a method for its use.
There is often need for drilling generally horizontal bores under roadways, railroads and similar areas where it is important that the surface not be disturbed for the placement of utility lines, water mains, gas gathering lines and the like. Such techniques have long been practiced and a variety of commercial drills have been developed for such purposes.
Typical commercially available, horizontal drilling devices utilize a carriage-mounted power source arranged to move longitudinally on a pair of rails or track. The power source rotates a spindle at its forward end which is connected to an auger having a bit or cutter head mounted on it. A longitudinal force is exerted on the auger to cause it to penetrate the earth by means of a hydraulic cylinder acting between the carriage and the track.
As the auger penetrates the earth, cuttings torn free by the cutter head are conveyed from the bore by action of the spiral auger flights. When the carriage has progressed to the limits of its travel along the rails or track, the auger is disconnected from the carriage and the carriage is retracted backwardly along the rails. A new auger section is then attached to that section within the bore and is connected to the carriage spindle. This procedure is repeated until the bore is complete.
A drill of the type described works well when used in drilling bores through compacted earth, typical of many roadbeds, or through relatively soft, homogeneous strata. However, when a drill of this sort is used in an attempt to drill a bore through a compacted, heterogeneous material such as a cobble or boulder-filled roadbed, great difficulties ensue. Upon striking a boulder or large cobble, the cutter head is often deflected causing the auger flights to enlarge the bore even to the point of causing the bore to collapse trapping the auger within the hole. If the deflection is very large, particularly if the cutter head is deflected either upwardly or downwardly, the hole must be abandoned and relocated. Cutter heads used with the typical auger drills do not have a practical capability of penetrating hard rock such as granites, tightly cemented sandstones and the like. All of these circumstances, cobble and boulder filled roadbeds, hard rock and the like, are common in portions of this country, particularly in the Rocky Mountain states. Consequently, there has developed a pressing need for drills which can bore straight horizontal holes through heterogeneous strata and hard rock.
Auger drills are also used in drilling vertical bores, particularly for soils testing, foundation work and the like, but more commonly rotary drills are used for this purpose as also are impact drills. Impact drills used for drilling vertical bores typically are powered by a compressed gas stream delivered to the drill through the drill pipe. The drill pipe and impact hammer may, in some instances, be rotated with the drill string during operation.
It has been proposed to provide a flow of liquid, water or other drilling fluid, to the bottom of the bore to aid in flushing cuttings from the hole when using an impact drill. This technique requires that the liquid and gas streams be separately conveyed down the hole with the air or gas stream passing through the impact drill while the liquid stream is directed to the working face of the bore. Such an arrangement has serious practical disadvantages. It requires a double-walled drill pipe or equivalent arrangement to convey separate streams of liquid and gas down the hole and it requires some method for directing the liquid stream around the drill and discharging it at the cutting face. Because of these practical difficulties, the advantages of combining rotary and impact forces to a drill bit and of combining an air or other gas stream to power the impact drill with a water or other liquid stream to condition the hole and to flush cuttings therefrom have seldom been realized.